1. Field of the Invention
The invention relates generally to downhole tools. More particularly, the present invention relates to progressive cavity pumps. Still more particularly, the present invention relates to progressive cavity pumps that are insertable and moveable through a tubing string disposed within a well.
2. Background of the Invention
A progressive cavity pump (PC pump), also know as a “Moineau” pump, transfers fluid by means of a sequence of discrete cavities that move through the pump as a rotor is turned within a stator. Transfer of fluid in this manner results in a volumetric flow rate proportional to the rotational speed of the rotor within the stator, as well as relatively low levels of shearing applied to the fluid. Consequently, progressive cavity pumps are typically used in fluid metering and pumping of viscous or shear sensitive fluids, particularly in downhole operations for the ultimate recovery of oil and gas. A PC pump may be used in reverse as a positive displacement motor (PD motor) to convert the hydraulic energy of a high pressure fluid into mechanical energy in the form of speed and torque output, which may be harnessed for a variety of applications, including downhole drilling.
As shown in FIGS. 1 and 2, a conventional PC pump 10 comprises a helical-shaped rotor 30, typically made of steel that may be chrome-plated or coated for wear and corrosion resistance, disposed within a stator 20, typically a heat-treated steel tube or housing 25 lined with a helical-shaped elastomeric insert 21. The helical-shaped rotor 30 defines a set of rotor lobes 37 that intermesh with a set of stator lobes 27 defined by the helical-shaped insert 21. As best shown in FIG. 2, the rotor 30 typically has one fewer lobe 37 than the stator 20. When the rotor 30 and the stator 20 are assembled, a series of cavities 40 are formed between the outer surface 33 of the rotor 30 and the inner surface 23 of the stator 20. Each cavity 40 is sealed from adjacent cavities 40 by seals formed along the contact lines between the rotor 30 and the stator 20. The central axis 38 of the rotor 30 is parallel to and radially offset from the central axis 28 of the stator 20 by a fixed value known as the “eccentricity” of the PC pump.
During operation of the PC pump 10, the application of torque to rotor 30 causes rotor 30 to rotate within stator 20, resulting in fluid flow through the length of PC pump 10. In particular, adjacent cavities 40 are opened and filled with fluid as rotor 30 rotates relative to stator 20. As this rotation and filling process repeats in a continuous manner, fluid flows progressively down the length of PC pump 10.
PC pumps are used extensively in the oil and gas industry for operating low pressure oil wells and also for raising water from a well. As shown in FIG. 3, PC pump 10 previously described disposed in a cased borehole 50 in a conventional manner to pump oil to the surface. Since PC pumps (e.g., PC pump 10) are often mounted tens or hundreds of meters below the surface, it is difficult to mount an electric drive motor to the PC pump. Consequently, as shown in FIG. 3, it has become common practice to secure the stator 20 on to the lower end of a string of production tubing 60. In particular, the upper threaded end of the stator housing 25 is axially connected end-to-end with the lower threaded end of the production tubing 60 with a mating threaded collar 65. Once the stator 20 is secured to the lower end of the production tubing 60, it is lowered into the cased borehole 50 on the tubing string 60. Thus, the production tubing 60 is used both to position stator 20 and PC pump 10 at a specific depth in the well bore, and to axially support the weight of the PC pump 10 and the weight of the fluid column extending between the PC pump 10 and the surface which bears against the upper end of stator liner 21.
Once the stator 20 is properly positioned at the desired depth for production, the upper end of the rotor 30 is threaded to the lower end of a sucker rod string 70 at the surface, lowered through the production tubing 60, and inserted into the stator liner 21. The rotor 30 is lower until the lower end of rotor 30 hits a tag-bar 80 extending across the lower portion of the stator 20. Once the lower end of the rotor 30 contacts tag-bar 80, the entire rod string 70 is lifted upward a predetermined distance to position the entire rotor 30 within the stator 20. To begin pumping, a drivehead at the surface applies rotational torque to the rod string 70, which in turn causes downhole rotor 30 to rotate relative to the stator 20.
One disadvantage of such conventional PC pumps and delivery methods is that the entire production tubing string 60 must be pulled from the cased borehole 50 to access, service, and/or repair the stator 20. Following service and/or repair, the stator 20 is reattached to the lower end of the production tubing 60 and lowered into the cased borehole 50, followed by the delivery of rotor 30 to stator 20 on rod sting 70. This process is time consuming, costly, and results in undesirable production delays.
FIG. 4 shows a conventional insertable PC pump 100 being disposed in the cased borehole 50. Insertable PC pump 100 is configured such that the entire PC pump 100, including the rotor 130 and the stator 120, is lowered into the production tubing 60 as a single package. As compared to the conventional PC pump 10 shown in FIG. 3, the stator housing 125 of insertable PC pump 100 is longer. In particular, housing 125 is sufficiently long to accommodate liner 121 in its lower portion, and accommodate rotor 130, axially spaced from liner 121, in its upper portion. A “no-go” assembly 190 is provided on the lower end of rod string 70 to prevent rotor 130 from being completely pulled from the stator housing 125. To lower PC pump 100 into the production tubing 60, the upper end of rotor 130 is secured to the lower end of the rod string 70, and stator housing 125 is hung from rod string 70 via no-go assembly 190. Then the entire PC pump 100 is lowered into the production tubing 60, the No-Go assembly 190 and rod string 70 supporting the entire weight of the PC pump 100.
Housing 125 is sufficiently long to permit rotor 130 to be axially pulled from liner 121, while still remaining within housing 125. This configuration allows rotor 130 to be pulled from of the stator liner 121 to flush the PC pump 100 without pulling the entire PC pump 100 out of the well. In some cases, housing 125 may be lengthened fifty feet or more to provide sufficient space to accommodate rotor 130 when it is axially spaced above stator 120. The additional length of housing 125 undesirably increases the weight and bulk of PC pump 100.
PC pump 100 is lowered to the desired depth at which an annular seating nipple, previously installed in the tubing string 60, is engaged by stator 120, thereby resisting the continued lowering of PC pump 100. In many conventional PC pumps, a locking or retaining mechanism (not shown) is provided between the stator and the seating nipple to lock and hold down the PC pump within the tubing string. However, such hold-down assemblies often require complex actuation, may become jammed or damaged, and add another degree of complexity to the PC pump assembly and installation.
Once stator 120 is properly seated and retained, continued lowering of stator 120 is prevented. However, rotor 130 may still be lowered within housing 130 until it is sufficiently positioned within the stator liner 121, at which time rod string 70 may be rotated to power PC pump 100. Any gaps or flow passages between stator 120 and the seating nipple reduce the effectiveness of PC pump 100 as they relieve the pressure differential between the ends of PC pump 100.
Accordingly, there remains a need in the art for improved insertable PC pumps and methods of delivering the same. Such devices, methods, and systems would be particularly well received if capable of being inserted into and moveable within into a tubing string, capable of being pressure tested to ensure a sufficient seal between the stator and the production tubing within which it is disposed, and capable of being handled and manipulated with relative ease.